Typescript: Avoid stripping class properties when a decorator is set (#8238)

The `babel-plugin-transform-typescript` removes class properties without value regardless if decorators are assigned to it or not.

packages/babel-plugin-transform-typescript/src/index.js

@@ -157,10 +157,6 @@ export default declare((api, { jsxPragma = "React" }) => {
 
       ClassProperty(path) {
         const { node } = path;
-        if (!node.value) {
-          path.remove();
-          return;
-        }
 
         if (node.accessibility) node.accessibility = null;
         if (node.abstract) node.abstract = null;
@@ -196,7 +192,10 @@ export default declare((api, { jsxPragma = "React" }) => {
         path.get("body.body").forEach(child => {
           if (child.isClassProperty()) {
             child.node.typeAnnotation = null;
-            if (!child.node.value) child.remove();
+
+            if (!child.node.value && !child.node.decorators) {
+              child.remove();
+            }
           }
         });
       },

packages/babel-plugin-transform-typescript/test/fixtures/class/properties/input.js

@@ -2,4 +2,6 @@ class C {
     public a?: number;
     private b: number = 0;
     readonly c!: number = 1;
+    @foo d: number;
+    @foo e: number = 3;
 }

packages/babel-plugin-transform-typescript/test/fixtures/class/properties/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": [
+    "transform-typescript",
+    ["syntax-decorators", { "legacy": true }]
+  ]
+}

packages/babel-plugin-transform-typescript/test/fixtures/class/properties/output.js

@@ -1,4 +1,8 @@
 class C {
   b = 0;
   c = 1;
+  @foo
+  d;
+  @foo
+  e = 3;
 }